The present invention relates to a method for producing microcapsules and, more particularly, to a method wherein polymeric capsule walls are formed by interfacial photopolymerization.
The formation of capsules consisting of a nucleus around which a shell-like coating of a polymeric or high molecular weight material is deposited is well known in the art. One of the most common methods for forming microcapsules is "complex coacervation" wherein two oppositely charged colloids, such as gelatin and gum arabic, are dispersed under carefully controlled conditions of pH and temperature to form a liquid wall around dispersed oil droplets and the liquid wall is subsequently hardened by physical and/or chemical action. See, for example, U.S. Pat. No. 2,800,457 to Green et al.
Various methods are also known for forming fluid-containing microcapsules by polymerization. For example, processes are known wherein fluid-containing microcapsules are prepared by dispersing a solution of a monomer and an oil in a non-compatible liquid to form droplets and, thereafter, the monomer is polymerized in the dispersed droplets to form a solid polymer capsule wall around each of the monomer droplets.
Processes are also known wherein the reactants forming the microcapsule walls are present in both the continuous and discontinuous phases and the walls are formed by an interfacial reaction. See, for example, U.S. Pat. No. 4,075,134 to Morehouse and U.S. Pat. No. 4,138,362 to Vassiliades et al. Urea-formaldehyde capsules are conventionally formed by sequentially adding urea and formaldehyde to an oil-in-water dispersion under conditions designed to provide a controlled reaction of the urea and formaldehyde which generates a polymer which encapsulates the internal phase. See, for example, U.S. Pat. No. 3,796,669 to Kiritani et al.
The formation of polymeric microcapsules by thermal polymerization and photopolymerization are also known. U.S. Pat. Nos. 2,969,330 and 2,969,331 to Brynko et al disclose a process for forming oil-containing microcapsules by thermal polymerization in which a polymerizable monomer is dissolved in an oil in which the polymer is not soluble and the solution is dispersed in an aqueous medium. The capsule walls are formed by introducing a polymerization catalyst into the aqueous dispersion while heating whereupon the monomer present in each oil droplet polymerizes and forms a shell-like structure which encapsulates the oil.
Japanese Kokai No. 52-140479 (1977) to Kansai Paint teaches a process for the production of water-containing microcapsules wherein an aqueous solution is dispersed in an oil-based photosensitive liquid composition including a photoinitiator, a nonionic surfactant, and an unsaturated compound to produce a water-in-oil emulsion. This emulsion is converted to a water-in-oil-in-water emulsion by the addition of an aqueous solution of a nonionic surfactant. By exposing the latter emulsion to the appropriate radiation, the oily interface is crosslinked to form the capsules. A related process for forming microcapsules by photocrosslinking the capsule wall is disclosed in Japanese Kokai No. 53-22581 (1978).
U.S. Pat. No. 4,021,364 to Speiser et al teaches a procedure for producing microcapsules of small diameter by the steps of dissolving a polymerizable compound in water together with the material to be encapsulated, dispersing this material in a hydrophobic liquid containing a surface active agent which results in the formation of micelles containing the monomer, and inducing polymerization by irradiation.
In order to produce microcapsules by coacervation or polymerization and obtain microcapsules having the desired size, impermeability, and strength characteristics on a uniform basis, each stage of the encapsulation process must be carefully controlled. Due to the large number of steps typically involved in controlling these reactions and the sensitivity to reaction conditions such as pH, concentration and temperature, prior encapsulation processes have not been entirely satisfactory and efforts have been undertaken to develop simplified and less sensitive procedures. While processes involving photopolymerization are known, for various reasons they have not been adopted for commercial applications. Consequently, there is still a need for a simplified method for producing microcapsules.